(a) Field of the Invention
The present invention relates to an interior permanent magnet synchronous motor, and more particularly, to permanent magnets that are embedded inside a rotor.
(b) Description of the Related Art
In general, a hybrid vehicle or an electric vehicle, often referred to as an environmentally-friendly vehicle, is driven typically by an electric motor (hereinafter, referred to as a “drive motor”) that produces torque from electrical energy.
Hybrid vehicles are capable of being driven in an electric vehicle (EV) mode, (that is a pure electric vehicle mode which uses only power from the drive motor to operate the vehicle), or in a hybrid electric vehicle (HEV) mode, (using both torque from an engine and torque from the drive motor as power) in order to reduce the amount of emissions which the vehicle produces and decrease fuel consumption. An 1 electric vehicle, on the other hand, is driven using only torque from the drive motor as power and has no other power source.
The drive motor, which is used as a power source for these environmentally-friendly vehicles, is often a permanent magnet synchronous motor (PMSM). The permanent magnet synchronous motor has a stator, a rotor which is disposed to have a predetermined air gap with the stator, and permanent magnets which are installed to the rotor.
Depending on a method of installing the permanent magnets to the rotor, there are for the most part two types of permanent magnet synchronous motors, a surface permanent magnet motor (SPMM) in which the permanent magnets are installed on a surface of the rotor and an interior permanent magnet synchronous motor (IPMSM) in which the permanent magnets are embedded in the rotor.
The surface permanent magnet motor does not produce reluctance torque because a saliency ratio, which is a difference in inductance between a D-axis and a Q-axis, is zero (the D-axis inductance and the Q-axis inductance are the same). However, the interior permanent magnet synchronous motor advantageously produces reluctance torque due to its saliency ratio, and thus the interior permanent magnet synchronous motor is more widely applied as a drive motor for a hybrid vehicle, an electric vehicle, or an elevator that requires high efficiency and output density.
In order to allow the permanent magnet synchronous motor to exhibit maximum performance under a restricted layout condition, it is necessary to maximize performance of the permanent magnets. Here, neodymium (Nd) in the permanent magnet improves intensity of the permanent magnet, and dysprosium (Dy) improves tolerance against high-temperature demagnetization.
However, these rare earth resources (Nd, Dy), which are constituents of the permanent magnet, are restrictively buried in some countries, such as China, and thus are very expensive, and fluctuation in their prices is significant. Recently, in order to cope with China's restrictions on rare earth resources, vehicle manufacturers have accelerated efforts to reduce usage of rare earth elements in the drive motor for an environmentally-friendly vehicle.
However, when a rare earth permanent magnet, which is applied to the drive motor for an environmentally-friendly vehicle, receives an opposing magnetic field that is too high for the magnet itself to withstand at a high temperature, irreversible demagnetization occurs in which magnetic intensity is lost.
This irreversible demagnetization occurs on a surface of the permanent magnet, which is affected by the opposing magnetic field due to a stator coil, that is, a rotor air gap direction surface that is directed toward the stator. This means that among the surfaces of the permanent magnet, the rotor air gap direction surface (i.e., the area between the outer surface of the rotor and the inner surface of the stator) is vulnerable to the irreversible demagnetization.
In addition, when the drive motor is continuously operated, an internal temperature of the permanent magnet increases due to an eddy-current loss, and in general, a temperature of the rotor air gap direction surface of the permanent magnet is also significantly increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.